Chen Haogang
2009-Jul-21 04:31 UTC
[Xen-devel] How does munmap() call Xen to decrease the reference counter when releasing a foreign page?
The ioemu-qemu-xen code uses xc_map_foreign_batch() to map pages from
a HVM domain. When the "map cache" goes full (on hash collision, more
precisely), it calls munmap() to release previously mapped address
space.
In Xen 3.3.0, xc_map_foreign_batch() finally calls
HYPERVISOR_mmu_update(), which traps to Xen to increase the reference
counter of the mapped machine page. However, it seems to me that
munmap() just silently set PTEs to 0 in Dom0''s kernel, without
notifying Xen to release the original page. Did I miss something? If
so, where is the code that inform Xen to decrease the reference
counter when a foreign page is being unmapped via munmap()?
The following is the call path of munmap() in Dom0''s kernel:
do_munmap
unmap_region
unmap_vmas
unmap_page_range
zap_pud_range
zap_pmd_range
zap_pte_range
ptep_get_and_clear_full
in include/asm-x86_64/mach-xen/asm/pgtable.h
274 static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
unsigned long addr, pte_t *ptep)
275 {
276 pte_t pte = *ptep;
277 if (!pte_none(pte)) {
278 if (mm != &init_mm)
279 pte = __pte_ma(xchg(&ptep->pte, 0));
280 else
281 HYPERVISOR_update_va_mapping(addr, __pte(0), 0);
282 }
283 return pte;
284 }
285
286 static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm,
unsigned long addr, pte_t *ptep, int full)
287 {
288 if (full) {
289 pte_t pte = *ptep;
290 if (mm->context.pinned)
291 xen_l1_entry_update(ptep, __pte(0));
292 else
293 *ptep = __pte(0);
294 return pte;
295 }
296 return ptep_get_and_clear(mm, addr, ptep);
297 }
In the above code, I believe xen_l1_entry_update on line 291 should
not be called since qemu''s mm->context.pinned is false. Also,
HYPERVISOR_update_va_mapping (line 281) is not called because qemu''s
mm != init_mm. All the other paths simply set the PTE through
assignment or exchange.
--
Best regards,
Chen Haogang
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